Add namespaces exclusion form

What does this MR do and why?

It adds namespace exclusion form to the namespace limits page, allowing to exclude a namespace from namespace limits/enforcement by selecting a namespace and adding a reason why it has been excluded.

Note: this MR depends on !119537 (merged) and !119737 (merged) being merged first.

Screenshots or screen recordings

before	after

How to set up and validate locally

• In bin/rails console run:

    Feature.enable(:namespace_limits_admin_dashboard)
    ::Gitlab::CurrentSettings.update(check_namespace_plan: true)

• Simulate saas and restart your gdk

  $ export GITLAB_SIMULATE_SAAS=1
  $ gdk restart

• Visit /admin/namespace_limits as root

• Open devtools to "network" tab

• In the form at the end of the page, select a namespace and type a reason for exclusion then submit

• The table should change into busy state and the request should return 200

• Try selecting the same namespace and fill the reason field and submit, you should see "400 Bad request - already excluded" error

• Try clearing out the form and submit empty fields, you should see "You must select a namespace and add a reason for excluding it" error

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #381681